In 1866 he was elected to the Cape Parliament to represent the electoral division of Piketberg and soon distinguished himself for his efficient and honest management of parliamentary procedure.

The Cape Colony had only recently been brought under responsible government, and parliamentary procedure was seen as being in need of reform and streamlining to accommodate an expanding territory, a rapidly growing economy, and an increased political independence from Britain.

With his characteristic industriousness, Tennant worked with the Molteno ministry to re-draw the rules of parliamentary operations to speed up the proceedings and remove potential for inefficiency and abuse.

Nonetheless, his characteristically gentle but firm impartiality meant that when he retired in 1901 (after having been Speaker for a total of nearly 22 years), he was greatly and widely praised by all of the various opposing factions in the Assembly.

Parliamentary author Ralph Kilpin, in his book "The Old Cape House", describes Tennant as having been a hard-working and imperturbable man, with a gentle and unaffected dignity.